1. Field of the Invention
This invention relates to the relative positioning of a first component movably mounted upon a second component. More particularly the invention relates to the coordination of the operation of coarse and fine actuators. such as coarse and fine tracking actuators used in optical disk drives.
2. Description of the Related Art
Optical disk drives, including compact audio disk players and computer data storage devices, employ a collimated beam of light to write to and/or read from a disk rotatably mounted therein. The beam is typically generated by a laser and passes through a series of optical components before the beam is used to change one or more local characteristics thereof. For reading data from the disk, the beam may be transmitted through or reflected from the disk such that the characteristics of the transmitted or reflected beam depends upon the local characteristics of the area of the disk with which the beam interacts.
Data is generally arranged on optical disks in either of two configurations. The first configuration is a series of concentric circular tracks. Random access to a particular area of the disk (and thus to particular data) is accomplished by radially seeking the optical beam across the tracks until the desired track is located, and then following such desired track until the particular area of the disk is reached. The second configuration is a single, spiral track. Random access to the spiral track is accomplished in a manner similar to that for circular tracks. The optical beam seeks across the boundaries of adjacent spiral track revolutions to radially locate the desired revolution of the spiral. and then follows such revolution until the particular area of the disk is reached. Hereafter. it will be understood that reference to "tracks" on an optical disk also includes the revolutions of a single, spiral track.
The tracks on an optical disk are randomly accessed and followed by a servo system which includes coarse and fine tracking actuators. The coarse tracking actuator may be, for example, a voice coil motor arranged to controllably move certain optical components radially (across tracks) with respect to the disk. The motion of the coarse actuator allows for access to any track on the disk. The fine tracking actuator may be a motor operatively coupled to controllably move some of the optical components also moved by the coarse tracking actuator. For example, the optical beam emitted by a laser may be redirected by a mirror or prism through an objective lens and into focused interaction with an optical disk. The coarse tracking actuator may drive both the mirror/prism and the objective lens radially with respect to the disk. The fine actuator may simply drive the objective lens, but not the mirror/prism, radially with respect to the disk. The motion of the fine actuator allows for access to a particular track within certain range of tracks In an alternative arrangement, the fine actuator may instead controllably tilt the mirror/prism. Registration of the optical beam to the tracks is optically sensed after the beam is transmitted through or reflected from the disk, as disclosed in U.S. Pat. No. 4,745,588.
In a simple tracking servo system, the fine tracking actuator compensates for track misregistration of the optical beam. Track misregistration may be caused, for example, by radial disk runout. If the track misregistration of the coarse tracking actuator is large enough the optical beam may be far enough out of its ideal path that certain problems arise. For example the path of the optical beam may not be aligned within a certain tolerance to the axis of the objective lens used to focus the beam upon a disk. The size and performance requirements of an objective lens in an optical disk drive are such that it can only be sized to adequately focus a properly positioned beam. If the offset or angle of the optical beam with respect to the objective lens or other optical components becomes too great the performance of the drive will degrade. A tracking servo system is thus most effective when the operation of the coarse and fine tracking actuators are coordinated to limit the size of the offset/angle.
By making the coarse tracking actuator closely follow the fine tracking actuator the optical beam is maintained in a nearly ideal path through the optical components. Such coordination requires accurate, stable sensing of the relative position of the optical components driven by the fine tracking actuator with respect to the position of the component driven by the coarse tracking actuator, and accurate adjustment of their relative positions. One technique for such relative position sensing is disclosed in U.S. Pat. No. RE29.963. The sensing system measures the average current provided to the fine actuator motor coil. The average current is related to the displacement of the optical components and is used to control the coarse tracking actuator. This technique is inexpensive but also inefficient, and thus fails to provide the bandwidths necessary for high performance optical disk drives.
Another perceivable technique for the aforementioned relative position sensing is to physically monitor the movement of the fine tracking actuator servo motor. However, the cost and complexity associated with the additional electronics required is not practical. Yet another technique for the aforementioned relative position sensing is disclosed in U.S. Pat. No. 4,864,552. The optical beam reflected from a disk is transmitted back through an objective lens driven by a fine tracking actuator, through a beam splitter and received by a split detector. The difference between the signals produced by each half of the split detector indicates the relative alignment of the objective lens with respect to the optical components driven by the coarse tracking actuator. This information is used to produce a drive signal which is then applied to the coarse tracking actuator to coordinate its operation with respect to that of the fine tracking actuator.
Two problems are associated with the determination of the relative position of the optical components by sensing the optical beam after it is reflected from a disk. First, an optical disk or similar calibration device must be present in the optical disk drive to align the optical components driven by the fine tracking actuator to those driven by the coarse tracking actuator. Second, the intensity of the optical beam used for relative position sensing is attenuated in its travel to and from the disk, and the intensity of the beam used for reading and tracking is attenuated by the beam splitter. In addition, this technique requires the use of an additional optical component, a lens which focuses the reflected optical beam onto the split detector.